4.1.2 Transmission of Communicable Pathogens Flashcards
What is disease transmission?
In order for a population of pathogens to survive, they must be able to successfully transfer from host to host
If pathogens are unable to find new hosts then they will go extinct
Disease transmission is defined as the transfer of pathogens from an infected host to an uninfected host
Transmission can be very risky for pathogens
During the infective stages, pathogens produce a large number of individuals to increase the likelihood that some will find a new host and survive
Describe transmission through contact with other people.
Some pathogens are transferred through physical contact between individuals
If the leaves of plants infected with Tobacco Mosaic Virus (TMV) touch the leaves of another uninfected plant, particles of the virus are transmitted
Sometimes individuals being within close proximity to each other is sufficient for transmission
The influenza viruses are spread in the air via tiny droplets of water. An infected individual breathes out droplets containing the virus and they are breathed in by an uninfected individual
Spores can also be involved in the transmission of pathogens
Spores are very small reproductive structures that are released into the environment. They are dispersed via wind or water
Once they reach a food source (host) they begin growing
Depending on the organism, spores can be produced via mitosis or meiosis so they can be haploid or diploid
P. infestans which causes potato blight produces specialised spores called sporangia. These structures are adapted for wind dispersal
Describe transmission of HIV/AIDS.
Human Immunodeficiency Virus is a retrovirus
The HIV virus is not transmitted by a vector (unlike in malaria)
The virus is unable to survive outside of the human body
HIV is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
This means HIV can be transmitted in the following ways:
sexual intercourse
blood donation
sharing of needles used by intravenous drug users
from mother to child across the placenta
mixing of blood between mother and child during birth
from mother to child through breast milk
Describe the transmission of tuberculosis (TB).
When infected people with the active form of the disease cough or sneeze, the
Mycobacterium tuberculosis bacteria enter the air in tiny droplets of liquid
TB is transmitted when uninfected people then inhale these droplets
TB, therefore, spreads more quickly among people living in overcrowded conditions
The form of TB caused by Mycobacterium bovis occurs in cattle but is spread to humans through contaminated meat and unpasteurised milk
Very few people in developed countries now acquire TB in this way, although meat and milk can still be a source of infection in some developing countries
Describe transmission via a vector.
Vectors are involved in the transmission of pathogens
A vector is any organism that transfers a pathogen from an infected individual to an uninfected individual
The vector themselves usually aren’t harmed by the pathogen
A lot of disease vectors tend to be insects
Insects are ideal vectors as they reproduce in large numbers which increases the likelihood of pathogen transmission
Describe the transmission of Malaria.
Malaria is caused by one of four species of the protoctist Plasmodium
These protoctists are transmitted to humans by an insect vector:
Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs
If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood meal
When feeding on the next human, Plasmodium pass from the mosquito to the new human’s blood
Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used
Plasmodium can also pass from mother to child across the placenta
What are the factors that affect disease transmission?
The transmission of disease ultimately depends on:
The presence of the pathogens
If the pathogen is not present in the population then it cannot spread
The presence of susceptible individuals
A high number of immune or resistant individuals in a population will reduce the likelihood of transmission
Animals and plants can be resistant to some diseases
Individuals who are resistant have mechanisms that prevent the infection or spread of pathogens within their body. They are not susceptible to disease.
These mechanisms are coded for by their genes
Individuals who are heterozygous for the sickle cell allele have resistance to malaria
Some humans are even resistant to HIV
Resistance is different to immunity
When resistant individuals are exposed to the pathogen for the first time they do not develop the disease and suffer no symptoms
An immune individual has been previously infected with the pathogen, suffered symptoms of the disease and recovered. They are highly unlikely to develop symptoms of the disease when exposed to the pathogen again
The proportion of resistant or immune individuals in a population influences the potential for transmission (as they are not susceptible to disease)
The higher the proportion, the lower the probability of transmission
Different types of disease transmission are affected by different factors
Human pathogens are affected by specific factors dictated by human behaviour and population size
Give examples of factors affecting transmission.
Pathogens that spread through direct contact or by droplet infection need potential hosts to be within close proximity to each other
Places or areas with high population densities are more likely to have high infection rates
E.g. cities and schools
Tuberculosis (TB) transmission is very high in places where many people have to sleep in confined quarters
E.g. poor housing and homeless shelters
Farmers who use monocultures to maximise yield and profit can experience large disease outbreaks
Farmers grow a large number of crop plants in a small area
As the crops grow the leaves of different plants touch each other, making the transmission of pathogens such as tobacco mosaic virus (TMV) very easy
The transmission of a pathogen through vectors can be affected by the biology of the vectors involved
Common disease vectors include mosquitoes and aphids
The population of vectors (usually insects) is influenced by weather and climate
What are the factors affecting the distribution of malaria?
Malaria is caused by one of four species of the protoctist
Plasmodium but these protoctists are transmitted to humans by an insect vector (female Anopheles mosquitoes)
The Anopheles mosquitoes favour habitats that have high rainfall, high temperatures and high humidity
This means malaria can occur where these mosquitoes are present and, as a result, is found throughout the tropics and sub-tropics (about 80% of cases are in Africa)
The Anopheles mosquitoes found in Africa also have longer lifespans and prefer biting humans than animals
In the 1950s, the World Health Organisation (WHO) coordinated a worldwide eradication programme. Whilst malaria was eradicated from some countries, the programme was mainly unsuccessful because:
Plasmodium became resistant to the drugs being used to try and control it
Anopheles mosquitoes became resistant to DDT and other insecticides being used against them
There is evidence that there are an increasing number of malaria epidemics due to climatic and environmental changes that favour the spread of the Anopheles mosquitoes
A social factor that has caused the number of cases of malaria in Africa, in particular, to increase in recent years is the increased migration of people due to war (when migration happens due to war the parasite can be transferred from areas that have the infection to new regions, and, if the Anopheles mosquito is breeding in the new region, then the mosquito vector will transfer the disease from one human to the next)
What are the factors affecting the transmission of human diseases?
The degree or level of poverty in an area often correlates with the transmission of human diseases in that area
Water-borne disease like typhoid, cholera and polio spread when human faecal matter enters and contaminates drinking water
Those below the poverty line usually live in areas with crowded housing with no sewage systems, sanitation facilities or water treatment facilities. In addition, many people in these areas have limited access to hygiene products
In the last 200 years, humans have spread across the globe, bringing their diseases and pathogens with them
The level of human movement and migration that currently exists means that populations are more connected than ever
In the past, the ocean and bodies of water would have acted as natural geographic barriers to prevent the spread of pathogens
The first flu pandemic in 1918 took one year to spread around the globe. The flu pandemic in 2009 only took 3 months to reach West Africa from North America
An individual can become infected in one country (where the disease exists) and get a flight to another country thousands of miles away. They may not show any symptoms until they have already arrived in the new country
The historical danger of human migration is well known
When colonisers arrived in the Americas they brought many European diseases with them, such as smallpox
The Native Americans had no immunity or resistance as they had never been exposed to these pathogens before (they were a fully susceptible population)
The invasive pathogens rapidly spread through the population causing a large number of deaths
The behaviour or cultural practices of humans can also affect the transmission rate of diseases
For example, in parts of Africa, it is a religious and cultural tradition to touch and kiss the dead. This was a major problem during Ebola outbreaks there and scientists had to work with the public to try and inform them that this tradition was increasing the spread of the virus
